ORA-28040: No Matching Authentication Protocol – Complete Oracle Database Troubleshooting Guide The ORA-28040: No Matching Authentication Protocol error is one of the most common authentication issues encountered after upgrading an Oracle Database, Oracle Client, or middleware application. It typically appears when an older client attempts to connect to a newer Oracle database that no longer supports legacy authentication protocols. This guide explains everything you need to know about the ORA-28040 error, including its causes, how Oracle authentication works, troubleshooting techniques, multiple solutions, best practices, and preventive measures. Whether you are an Oracle DBA, System Administrator, Developer, or Infrastructure Engineer, this article provides a practical, production-ready approach to resolving the issue safely. What is ORA-28040? The ORA-28040 error indicates that the authentication protocol used by the Oracle client is not compatible with the authenticat...